1. Field of Invention
The present invention relates to instruments for detecting microbial growth and, more particularly, relates to optical means to measure spectral changes of dyes due to metabolic activities of the microorganisms.
2. Description of Prior Art
It is desirable to test various industrial samples such as food, pharmaceuticals, cosmetics and water for microbial contamination. One area of biological testing of samples involves the estimation of the total number of bacteria, yeasts and molds as well as concentrations of specific groups of organisms within the material. One widely used method is known as the "standard plate count" method and involves culturing a diluted sample of product in an agar growth medium. The plates containing the sample and the growth medium are incubated for 24 hours to 5 days depending upon the assay. After incubation, colonies of microorganisms which have grown in the agar are counted.
Colorimetric methods have been successfully used to classify microorganisms in clinical samples. These methods are used with pure cultures Without interfering substances. In actual industrial samples, however, solid material can mask the optical signal yielding poor signal to noise ratio of the detection system. In order to resolve this problem a special vial configuration was designed. U.S. Pat. No. 5,366,873 to Gideon Eden and Ruth Eden (Nov. 22, 1994) discloses a device for detecting microbial growth from a sample substance wherein the device includes a container which is at least partially transparent. A fluid layer is disposed in a container for cultivating microorganisms therein. An indicator substance is disposed in the fluid layer for undergoing transformation in the presence of microorganism growth. A barrier layer is disposed in the container which is a semi fluid substance, the fluid portion of which is the same composition as the fluid layer in which the microorganisms are cultivated. Therefore the fluid in the semi-fluid layer is in equilibrium with the fluid layer. The semi-fluid substance provides a barrier to solid substances introduced into the fluid layer while providing a zone within which changes in the indicator substance due to microbial growth can be detected.
The patent also discloses an embodiment of an instrument comprising a light source positioned at the bottom part of the container such that the transmitted light is directed through the transparent part of the vial and the barrier layer. A simple and inexpensive light source is a Light Emitting Diode (LED) which is available in various wavelengths in the range of red, orange, yellow, green and blue spectra. The LEDs are controlled by a controller which provides stable electrical energy.
Light Emitting Diodes are quite desirable for specific applications. If a limited number of discrete spectrum bands need be measured, then LED's can be adequately utilized, especially in situations where multiple samples are measured simultaneously. Rather than using a mechanical indexing system, in which a single light source (as well as a single detector) travels to each sample location, one can provide a dedicated light source (as well as a dedicated detector) located in each sample's position. LED's are reliable devices, quite inexpensive and can provide long t;me operation, eliminating expensive and fragile mechanical apparatus. There are, however, serious disadvantages to LED's when utilized in commercial systems. Most of the problems are associated with their scattered parameters. When using commercial LED's, which are manufactured in huge quantities, it is common to find LED's which differ widely in their light characteristics. Differences of up to 50% of light energy can be found in "identical" LED's. The transmitted light angle can also vary considerably, changing the distribution of light at specific angles. In addition the LED/sensor combined characteristics may vary, depending upon the relative position in which they are mounted. U.S. Pat. No. 5,164,796 to James L. Di Guiseppi and Thurman C. Thorpe (Nov. 17, 1992) attempts to correct these differences by employing a stabilized current source connected to a serial combination of groups of LED's. While identical current applied to different LED's can reduce the light variations it still can result in significant differences. In addition, this arrangement cannot compensate for the angle differences and the mounting variations among the different locations.